This invention relates to a gas sensor having a sensitive element in the form of a laminate of relatively thin layers on a plate-shaped substrate which is enclosed in a generally cylindrical hood formed with gas inlet and outlet openings.
Currently various kinds of gas sensors are used in many fields. For example, in the automobile industry it has been popular to use either an oxygen sensor or a carbon dioxide sensor in an air-fuel ratio control system as a means for producing a feedback signal indicative of an actual air-fuel ratio of a gas mixture supplied to the engine.
In most cases the sensitive part of such a gas sensor has a layer of either a solid electrolyte or a semiconductive metal oxide as the essential element of the sensor with the provision of some other layers such as electrode and shield layers, and it is a recent trend to construct the sensitive part in the form of a laminate of relatively thin layers on a substrate to thereby reduce the size of the sensor and enhance the sensitivity. For practical reasons such as the ease of production, it is usual to use a plate-shaped substrate so that the sensitive part as a whole becomes generally plate-like.
In a practical gas sensor, the plate-shaped sensitive part or element is fixed to one end of a sensor body, and a hollow cylindrical hood is attached to the sensor body so as to enclose the sensitive element therein for the purpose of protecting the sensitive element from shocks or collisions during handling and also from high temperatures and high flow velocities of gases subject to measurement. To allow a portion of a gas subject to measurement to come into contact with the sensitive element, two or more openings in the form of slits or holes are formed in the cylindrical side wall of the hood. With a view to maintaining good responsiveness of the sensitive element, these openings are located such that the gas which flows into the interior of the hood collides directly against the sensitive element. In other words, a cross-sectional plane bisecting the openings intersects the senstive element.
In general, the output characteristic of a plate-shaped gas-sensitive element exhibits some changes as the orientation of the element with respect to the direction of the flow of a gas subject to measurement varies. That is, the element exhibits directionality. For example, we have recognized that, when an oxygen sensor having a plate-shaped sensitive element is employed in the aforementioned air-fuel ratio control system, the accuracy of the control depends to some extent on the orientation of the oxygen-sensitive element with respect to the direction of the flow of exhaust gas at the location where the sensor is disposed. From a practical point of view, it constitutes a significant inconvenience that the sensor must be installed in the exhaust system in a particular orientation.